Acoustic decremeter



April 3, 1934. E, DlETZE El- AL 1,953,157r

ACOUSTIC DECREMETER Filed Sept. 25, 1931 Fg. y

loudspeaker v ATTORNEY N UNITED Eginhard Dietz STATES VPAJIENT OFFICE 1,953,157 ACOUSTIC DECREMETER ale, Jr., East ican Telephone and e, Westfield, and Walter D. Good- Orange, N. J., assignors to Amex'- Telegraph Company, a corporation of New York Application September 25, 1931, Serial No..565,180 12 claims. (o1. 181-05) This invention relates 'to and more particularly to a mining the rate of decay o beration period in halls and acoustic decremeters, decremeter for deterf sound or thereversimilar places.

Itsobject is to provide a decremeter which can be operated while the far as movementof occupa to do this in spite of such be present in the room.`

roomv is in normal use so nts is concerned, and general noise as may Another object is to render possible the measurement of the decay or damping constant, or

reverberation devices.

period, without the use of timing Still lanother object is to devise a source of 15 sound with which no appreciably fixedywave pattern is set up gation. l l

` The reverberation defined as the time require sound in a room to decay intensity or volume. tht reverberation period is frequencies, ithas become in the room or rooms under investiperiod of a hall is usuallyd for the intensity of l to 10-6 of its initial In view of thefact that different for different common to establish a source of a denite frequency and find the time required for this to fall to a given volume and to repeat this for other frequencies.

There are at least two denite difficulties encountered in such f measurements.

The first of these is that there is present a level of noise which frequently is 0 above the level to which it is desired thatthe sound under study should be allowed to fall unless one starts with excessively large initial volume.

The second is that for-a not e of a given frequency a definite wave pattern is set up in the room, and

the

tions of theisource of sou results obtained Will depend upon the posind and the receiving device, and this wave pattern is altered by movement ofl persons` or of apparatus in the room.

Attempts to overcome the 40 ties usually consists in first of these difficulisolating the room so as to keepout all extraneous noises, and the second is usually reduced by varying the frequency 4in a definite way or by having some moving article in the room which continual l5. pattern. These have the ly changes ,the Wave disadvantage, however,

,that the measurments must be taken under conditions of undue inconvenience to the occupants,

or under conditions with which the reverberation period will not be the same as that of the room when in` normal use.

To accomplish the objects of this invention our acoustic decremeter comprises essentially a Warbling source of sound; Athat is, a composite note of -a group orf band of fre -ji5 amplitude irregularly with quencies changing in respect to each other so that the wave pattern is continually changing.

Cooperating with this is a receiver of room sound followed by amplifiers and by a band-pass filter which permits only the passage of that portion of the sound spectrum which is being studied, and 60 thus discriminates strongly against the noise normally present in the room. The output of the filter passes to a rectifier of special characteristics and then to a differential galvanometer. In

addition, an electrical discharge circuit of adjustable time constant comprising a condenser and a resistance, is connected to the galvanometer. The circuits are so adjusted that when the source of sound is being activated and current` ris flowing through the resistance, the galvanometer deflection is zero. By deactivating the sound source and opening va'battery supply to the discharge circuit, the sound continues to arrive and activate the galvanometer, but with a definite decay rate, and the condenser discharge through the resistancealso falls off with a definite decay rate. When properly adjusted the two effects balance and no galvanometer deiiection results. The decay constant of the discharge circuit, which can readily be calculated, is then equal to or simply related to that of the room, and consequently, the period of reverberation may be calculated without the necessity of any measurements oi time.

The invention will be better understood by reference to the following specification and accompanyng drawing, in which Figure 1 shows in block form the elements comprising our decremeter; Fig. 2 shows an oscillating circuit for obtainingV a Warbling source of sound; Fig. 3 indicates 90 thelform of output of such a source, and Fig. 4 shows in further detail the circuit of our decremeter.

Referring more specifically to Fig. 1', there is shown at 5 an oscillatory with a modulated out- 95 put which supplies a loud speaker 6 constituting a source of warbling sound; that a band of frequencies changing irregularly in amplitude. This sound is then received by a pickup device 8 and amplified by a variable gain amplifier 10. The latter may be the amplifier cfa sound meter or volume indicator. vConnected'to the output of `this amplifier is a band-pass filter 13 adjusted to pass frequencies ,over a limited range only and constructed so that this region may be altered to any portion of the spectrum desired. The output of the band-pass filter is now passed throughA a rectifier 15, this rectifier being one of a type which has a uniform exponential characteristic throughout the working range. The rectified current is then allowed to pass through a galvanometer 17, preferably one of the zero center type, such that deflection in either direction will vanometer 17. -Th direction and magnitude of the voltage is adjusted so as to neutralize the deflection in the galvanometer while the loud speaker is being activated. An operating key 21 is provided such that, at any desired moment, the loud speaker may be deactivated. This key at the j made,it will be apparent that the such a source isset up,

same time removes voltage from the discharge circuit through the galvanometer which then proceeds to discharge with a definite decay rate determined by the resistance and capacity of the circuit. At the same time the sound which is still present in the room continues to send cur-'- rent through the galvanometer, but at a definite decay rate determined the room. By suitable adjustment of the constants of the discharge circuit, the two currents through the galvanometer will be of the same magnitude, and will decay` at the same rate, this condition being indicated by the fact that there is no deflection, momentary or permanent, of the galvanometer. The time constant of the electrical circuit can readily be calculated and, under conditions for which adjustments have been "time constent or decay rate for the room will be proportional to that of the electric discharge circuit, and bysimple transformation this may be converted into the reverberation period for the room. Having made the determinations at one frequency a proper operati on of a selector key 23 will then change the frequency of the oscillator 5 and also correspondingly change the transmission band of the filter'13, whereupon determination of the constant at this new frequency may be made. Further details of the operation of this circuit will be given in connection with the description of Fig. 4.

Reference has been made to the importance of having a source of sound which shall not set up a fixed wave pattern in the room. We overcome the difficulty by use ofa special form of vacuum tube oscillator which has a modulated output, preferably of an irregular form. When however, it will be found to consist not only of the natural frequency of the oscillator, but of numerous harmonics spread throughout the audible frequency spectrum. Such a source is shown in Fig. 2 to be described p below. Itis essential, however, to limit the transmission of soundto the galvanometer to a small portion of the spectrum only, and it is in part for this reason thaty we introduce the band filter shown at 13 in Fig. 1. Asecond important function of the band filter isto dscriminate against the noises .which are present in a room when in normal use, and the presence of such a band fllter makes it feasible to carry on measurements Ain a room in the presence of such general noise,

permitting the decay of that portion of the sound which is being studied to proceed to a much further degree without the confusion which would arise if all room noise were transmitted through the receiving system. Thus, there is a definite cooperation between the source by the characteristics of vof the resistances and the tuned high frequency .is established. This is warbling sound, the noise of the room, andthe band filter in the receiving circuit.v

Referring more specifically now to Fig. 2, there is shown one form of modulated oscillator which we find particularly satisfactory. This comprises a vacuum tube 30 lhaving two grids, the one grid is connected to filament through the tuned. circuit comprising condenser 31, and wind- This circuit ment is provided by. the third winding 36 of the transformer. The plate of the tube is also con-- nected to filament through B-battery 40 and primary of transformer'41, the secondary of which supplies the loud spea uating device 42 by means of which. the volume of sound delivered by the loud speaker may be controlled( The bias of the first gridmentioned is controlled in part by a resistance 43, and that of the second grid by means of the condenser 44 and grid leak 45, and'flnally, the coupling of the output to the input circuits may be controlled by the feed-back resistance 46 and condenser 47' which latter is inserted to block the'flow of di- -rect'current through its portion of the circuit. Such a circuit has the property that it will commence oscillating at the high frequency but will be periodically blocked at a frequency determined by the low frequency circuit. Also, the constants other elements, are such as to give large distortion, so that in addition to the oscillations' of frequency for whichrthe circuit is tuned, there will be numerous harmonics, many of these being of a frequency which'differ from frequency.

The form of output which we derive from such by a multiple of the low er 6 through an atten-A plotted against time as absciss and extending l over a length a-b, the time of one cycle for the low frequency circuit. It will be observed that' during a portion lof this cycle the oscillator is blocked, and during the rest of the time a wave of very irregular amplitude or irregular envelope a'. condition which we find particularly good for satisfactory results in measuring the characteristics of 'a room. The filter which should cooperate with such a source is, one which will exclude all frequencies except those in the neighborhood of the 'band to be studied. It is important that the signals' passing throughl vthe receiving system shall not'be restricted to a single frequency such as would be the case were a sharply tuned circuit to be used, for the system would then behave as though a single frequency were being emitted and were `setting up a definite wave pattern in the room. It is for this reason A that 'a band filter of, a-deilnite width is desired. If the frequency of the' oscillator is, `say 1000.. cycles, then the filter could very well be one passing the band from 900 to 1100 cycles.

Referring now to Fig. 4, to obtain a better understanding of certain features of our invention. the portion A corresponds to the circuit described ias' in connection with Fig. 2. In this case. however,

the oscillator is of a somewhat vmodified form com prisng as it does, two tubes in tandem, the one of which 4B is the low frequency oscillator, and

theotherof which C is the high frequency oscillator. The first of these uses'a series inductive feed-'back to a tuned grid. 'This in an overloaded grid leak oscillator, and provides a distorted wave shape containing a high percentage of harmonics. The third winding of the transformer `is in series with the grid circuit of the oscillator C. This oscillator C is tuned to the mid-frequency of the operating band; say 1000 cycles, and is modulated by" the lowfrequency oscillator.' The n type of grid cut-off modulation employedfor this second .oscillator provides a circuit in which the grid bias depends von the current flowing through the resistance in series with the grid. 'I'he bias itself is not sumcient to block the grid but is reenforced by the voltage from the iirst oscillator to obtain this blocking action. The oscillator B,

, formance of the oscillator.

consequently, has control over the oscillator C, both as yregards the blocking action which materially stabilizes the two, andthe forcing of the tube to start more rapidly into oscillation after blocking. from this duces the eifect'of tube. The increased stability alsoreload impedance on the per- -Leakage around the blocking condenser 56 is provided by the third winding of the' transformer 50. Obviously, in case this leakage is not sufficient, a grid leak of appropriate value could be shunted around the condenser56. A separate transformer 59 feeds the modulated output to the loudspeaker 6 through a balanced constant impedance attenuator 60.

ially satisfactory. It will as shown at 23 ingFig. l.

- throughout thev operating range. which we have found particularly satisfactory is In general, it will be desirable to find the decrement at a number of diiferent frequencies such, for instance, as 500 cycles, 1000 cycles and 1500 cycles. Accordingly, additional condensers 61 parallel to the condenserv'l may be provided to alter the frequency of the oscillator, and while we have shown .but one such condenser 61 it is understood that there will be as many as necessary to provide for as many different frequencies as may be desired. 'I'hese condensers may be introduced to the circuit by appropriate switches as shown in the drawing. Bridged across the terminals of the loud speaker 6 is a circuit 62 by means of which the loudspeaker maybe shortcircuited. -v

The band-pass filter may be of any suitable form, the one which is shown herein being espebe observed thatc it consists of two sections vof the ladder type of nlter. In'l the first section the shunt element consists of capacity, and in the secondsection the shunt element consists of inductance, this filter having the property of giving equal attenuation on both sides of the band. As shown in the figure, the band which is passed by the filter in the frequency of the oscillator C. And, in practice, we would find it desirable to operate all the' necessary switches with one mastergkey The output of the filter is suppliedto the primary of transformer through a variable impedance matching network 71 The secondary of the transformer is connected to a full'wave rectifier 73 ofthe ex`- of a type'fin vponential type, and in particular which` the exponent shall remain constant One form thus materially increasing the output a so-called rectox, or copper oxide rectifier, which follows closely a square-law curve over a wide range. The rectifier output is supplied to the galvanometer 17. Also bridged across this galvanometer 17 isa the condenser, 74 and variable resistanceA 75. Voltage is supplied to the discharge circuit by battery 77 and potentiometer 79 through contacts of the relay 81.

The operation of the circuit may now be described as follows: Upon closing the key 82 of the relay 81, the short-circuit across the loud speaker 6 is removed, and the room becomes filled with an irregular warbling sound which is picked up by the receiver 8, amplified, filtered and applied to the rectifier 73. The rectified component therefrom iiows through the .galvanometer 17 tending to produce a deiiection. The operation of the relay 81, however, closed circuit for battery 77 through the potentiometer 79, and also connected battery to one side of condenser 74. Thus, the condenser 74 is charged to a voltage given by the drop through a portion of the resistance 79. I'he current from the battery 77 through the galvanometer 17 flows'through the potentiometer 75. The purpose of the potentiometer 75, which is varied simultaneously with the potentiometer 79, is to keep the current through the galvanometer 17 at approximately the same value independent of the setting of potentiometer 79. The direction of the current is so poled as to give a deflection in the opposite direction from that due to the sound source.

By adjustment of the gain of amplifier 10 these deflections may be made equal so that the galvanometer shows `zero deflection. With this adjustment the key 82 is now`released, whereupon the loud speaker 6 is deactivated and battery is removed from the condenser. The sound in the room still continues to be received at 8 causing a current to flow through the galvanometer with a definite decay rate. The condenser 74 also discharges with its definite decay rate, and by adjustment of the resistance thesel decay rates may be made equal to each other so that the galvanometer shows no deflection during the period of decay. This condition exists at least up to the time when the sound in the room is decayed to approximately that of the noise level of the room, whereupon a continued current will flow from the rectifier. But in general, the adjustments of the attenuator 60 and amplifier 10 are such that the desired adjustment is obtained? before this conditionv has been reached, and in any event it will be so small as to be near the lower limit of sensitivity of the galvanometer.

discharge circuit comprising The feature of having a rectifier with a univ then the output current is approximately the as to shift the band in accordance with the same for an impressed sine wave or for a complex voltage having equal effective values, and is given by limit ltaand where K2 is a proportionality factor. -If the'receiver 8 is either a velocity operated or a pressure operated device, then the out-e0 where K1 is a proportionality factor and S is the sound intensity at time (t) When a sound source is cut-off in a room the sound decays according to the relation e for live rooms (3) so and A v`=e"|l(1-) M for dead rooms (4) Where Su is a sound intensity at time zero a=the average absorption coefficient for the room.

'may

N;total area of walls, ceiling and fioor.

V=volume of room.

Both expressions are of the form S -A g3 e (5) Reverberation time T is defined as the time during which' the originalsound intensity is reduced by 10a when the source is cut off, hence above equation. Hence, the rectifier output is from (1) I --Ioer-4t (8) The current at any instant during the discharge of a condenser C through resistance R is given by i s Il Ion-Ilo Substituting Equations (8)' and (9) in (10) we obtain the condition l A=RC (11) whence Thus, it is seen, that a. determination of the reverberation .period is obtained through the measurement of a resistance and a condenser.

It" should be pointed out that it is not necessary to have a rectifier obeying square-law relationship. The only condition is hat the rectifier shall be an exponential one, the exponent remaining a constant. If, for instance, the Equation (2) had been of the form I =K2Eb (13) where b is a constant, the treatment would have been the same and Equation (11) would have taken on the form from which again the period of reverberation may be readily obtained.

It will be observed that a system has been devised which can be made in a very compact form and thus be readily portable. Also, that the operation has been reduced to one of great simplicity requiring the operation of but two keys, one the frequency selector key as shown at 23 in Fig. 1, and the other the operating key 21. The introduction of the filter makes it possible to take readings in a room in spite of high noise level, it being only necessary 'in such event to decrease the loss in the attenuato'r 60 to such an extent that the volume of sound given off by the loud speaker shall exceed that of room noises by. a certain amount. By the introduction of a corresponding amount of attenuation in the amplifier 10, the signal volume at the rectifier may be kept to substantially the same level, and the necessary 4ratio of signal to noise be maintained. In general,

we find it desirable to operate the loud speaker 6 at as low a volume as is feasible, and still give reliable readings, that is, no deflection of the galvanometer in a length of time corresponding to an appreciable portion of the reverberation period.

What is claimed is:

1. In an acoustic decremeter adapted to cooperate with a warbling source of sound, the method of finding the acoustic decrement of a room which consists in electrically receiving and amplifying the sound, filtering out all frequencies except those` to be measured, rectifyingl the electrical sound wave, and comparing the rectified sound decay current with an electrical discharge decay current.

2. In an acoustic decremeter, the method of measuring the reverberationI period which consists in generating a warbling source of sound 4in the presence of noise, electrically receiving and amplifying the sound and noise, filtering out vallfrequencies except those to be measured, rectifying the electrical sound wave, balancing the rectified current against a constant direct current, deactivating the source of sound, and balancing the decaying sound current against an electrical discharge decaying current.

3. In an acoustic decremeter adapted to cooperate with a source of sound, the combination of a' reciever and a rectifier of uniform exponential;characteristics connected to said re-- ceiver, an electrical discharge circuit of known damping constant, and means connected to the said rectifier and alsoA to said discharge circuit for comparing the rectifier current with the discharge circuit current.

4. In an `acoustic decremeter adapted to cooperate with a source of sound, the combina-` tion of a receiver and a rectifier of uniform exponential characteristics, an electrical discharge circuit of known damping constant, a differential galvanometerassociated with the rectifier and with the discharge circuit, and adapted to comfpare the two currents.

5. In an acoustic decremeter adapted'to cooperate with a source of sound, the combinaf tion of a receiver and a rectifier of uniform ex- '4 to a Value such that the two circuits feeding into the galvanometer yield a zero deflection.

6. In an acousticv decremeter adapted to cooperate with a source of sound of variable frequency, the combination of a receiver, a wave lter and rectifier feeding into a differential galvanometer, anelectrical discharge circuit of known damping constant also feeding into the galvanometer, and means for adjusting the damping constant of the discharge circuit to a value such that the two circuits feeding into the galvanometer yield a zero deflection.

V'7. In an acoustic decremeter adapted to cooperatewith a warbling source of sound, the combination of a receiver, a rectifier, a wave lter connected between said receiver an'd said rectifier, anv electrical discharge circuit of known damping constant, and means connected to said rectifier and said discharge circuit for adjusting the damping constant of the discharge circuit to the damping constant ofthe rectifier output.

8. In an acoustic decrerneter adapted to cooperate with a warbling source of sound, the combination `of a receiver, a Wave lter and rectier of uniform exponential characteristic, an electrical discharge circuit of known damping constant, a galvanometer associated with the rectifier and discharge circuit, means lfor supplying voltage through the discharge circuit to the galvanometer of such magnitude as to neutralize in'the galvanometer the current from the rectifier while a source of sound is activated, and means for adjusting the time constant of the discharge circuit to a value such that the decaying current of the discharge circuitis of the same value asthedecaying current from the rectier.

9. In an acoustic decremeter adapted to cooperate with a warbling source of sound, the combination of a receiver, a rectifierl and means connected between saidreceiver and said rectifier for discriminating against room noise, an electrical 5 discharge circuit of known damping, and means connected to said rectifier and said discharge circuit for adjusting the damping constant of the discharge circuit to the damping constant of the rectifier output. f

10. In a system for determining acoustic decrement, a warbling source of sound, means for altering the frequency level of the sourceof sound, a r

receiver for said sound, a lter forV passing a predetermined portion of the incident sound and meansfor altering this lter in accordance with the alteration of the frequency level of the sound source, arectifler associated with the receiver, an electrical discharge circuit of constant, and means for comparing the damping constant of the discharge circuit with the damping constant'of'the rectier output.

11. In a system fordetermining acoustic decrement, a source of warbling sound comprising a vacuum tube oscillator of relatively high frequency, a vacuum tube oscillator of relatively low frequency so associated that the first named oscillator acts to cause blocking of itself at afrequency equal to that of the second oscil1ator, a receiver of said warbling sound having associated therewith av rectifier of uniform exponential Vcharacteristics, an electrical discharge circuit of known damping constant, and means for comparing the rectier current with cuit current,

12. In a system for determining acoustic decrement, a source of warbling sound comprising a vacuum tube with two grids, a high frequency tuned circuit. across one grid input, a low frequency tuned circuit across the second grid input, a coupling connection from the plate circuit to each of the tuned circuits, thev coupling so ,ad-u justed that the tube oscillates at high frequency but blocks periodically at low frequency, a receiver adapted to receive said warbling sound and having associated'therewith a wave filter and rectier for selecting and rectifying a predetermined portion of said warbling sound, an electrical discharge circuit of known damping constant, and means for comparing the rectier current with the discharge circuit current.

EGINHARD DIETZE. WALTER D. GOODALE, JR.

the discharge cirknown damping f 

